A cathode ray tube (“CRT”), one of typical display devices, has been extensively used in television sets or computer monitors, but fails to catch up with the recent trend of miniaturization and lightweight of electronic equipments.
Thus, a variety of technologies have been developed in an effort to replace the cathode ray tube with new display devices, examples of which include a liquid crystal display (“LCD”) using an electric field optical effect, a plasma display panel (“PDP”) using a plasma discharge and an electroluminescence display (“ELD”) using an electric field light-emitting effect.
Among these devices, the liquid crystal display, which features thin lightweight configuration and low electricity operability, is showing rapid expansion in its range of applications with the improvement of liquid crystal materials and the development of fine pixel processing techniques, and is widely used in household television sets, desktop computer monitors, notebook computer monitors, large-sized flat panel television sets and so forth.
Most of the liquid crystal displays require the use of a separate backlight unit that serves as a light-flatting element for regulating the quantity of an incoming light to display images.
As shown in FIG. 1, a liquid crystal display module 1 for use in typical liquid crystal displays is comprised of a liquid crystal display panel 2 filled with liquid crystal, polarizing plates 4a and 4b for polarizing a light directed to the upper and lower surfaces of the liquid crystal display panel 2, a backlight unit 6 for supplying an uniform light to the liquid crystal display panel 2, a main support 8a for maintaining an external configuration of the liquid crystal display module 1, and a top case 8b. 
Unlike the cathode ray tube or the plasma display panel, the liquid crystal display panel 2 does not emit any light by itself but merely changes orientation or arrangement of the liquid crystal. This makes it necessary to provide, at the rear of the liquid crystal display panel 2, the backlight unit 6 for evenly surface-irradiating the light on an information display surface.
In this regard, the backlight unit 6 is classified into an edge type and a direct type depending on the position of a light source. As illustrated in FIG. 2A, the edge type backlight unit includes a light source 12 disposed at one edge of a light guide plate 14 for surface-irradiating a light. In contrast, the direct type backlight unit is subdivided into a dot type wherein a plurality of dot-like light sources 16a are mounted on a substrate 30 as shown in FIG. 2B and a line type wherein a plurality of linear light sources 16b are mounted on a substrate 30 as shown in FIG. 2C. In such direct type backlight units, the light sources are substantially evenly distributed on the entire surface of the substrate.
Examples of the light source conventionally used include an electroluminescence (“EL”) element, a cold cathode fluorescent lamp (“CCFL”) and a hot cathode fluorescent lamp (“HCFL”). In recent years, extensive use is made of a light emitting diode (“LED”) that has a broad area of color reproduction and is environmentally friendly.
Research has been made to develop methods of using the light emitting diode as a light source in the backlight unit. Subjects of the research include a method of taking advantage of a blue color light emitting diode and an yttrium aluminum garnet (“YAG”) fluorescent body, a method of using an ultraviolet emitting diode in combination with fluorescent bodies of red, green and blue colors, and a method of employing red, green and blue light emitting diodes to admix the lights generated from them.
The method of taking advantage of a blue color light emitting diode and an yttrium aluminum garnet (“YAG”) fluorescent body is disadvantageous in that the light source thus produced has a reduced ability to express the red color and a low light emitting efficiency. Likewise, the method of using an ultraviolet emitting diode in combination with fluorescent bodies of red, green and blue colors poses a drawback in that it is difficult to develop the fluorescent bodies, with the resultant light source exhibiting a deteriorated thermal characteristic.
The method of employing red, green and blue light emitting diodes is effective in designing the light source to have a broadened range of color reproduction, thank to the increased intensity of red, green and blue lights emitted from the respective light emitting diodes. However, the method has a problem in that it is difficult to compose a combination of diodes for a white surface light source.
In the meantime, along with the recent trend of pursuing a large-sized and high image quality display device, a demand has existed for a backlight capable of outputting a high flux light. In order to comply with such a demand, there have been developed lenses for collecting lights emitted from light emitting diodes, semiconductor chips and diode materials.
A typical light emitting diode, one of solid semiconductor devices that convert electric energy to light energy, includes doping layers and an active layer. If a biasing voltage is applied to two oppositely positioned doping layers, electron holes and electrons are injected into the active layer and recombined to generate a light. The light generated in the active layer is emitted in all directions and escaped from the light emitting diode through every surface exposed to the outside. The light escaped from the light emitting diode is oriented to a desired direction by means of a backlight unit that incorporates the light emitting diode.
However, the light emitting diodes developed thus far cannot provide a sufficiently great light emitting efficiency, due to the light loss when penetrating a current diffusion layer and the light loss caused by a total reflection at a boundary surface.
Accordingly, in a backlight unit requiring an output of a high flux light, it is inevitable either to apply an increased current to a light emitting diode or to increase the number of light emitting diodes used.
In the case that an increased current is applied to a light emitting diode, a great deal of heat is generated from the light emitting diode, thus reducing the light emitting efficiency and making it necessary to add a heat radiation design to a substrate on which the light emitting diode is mounted. In the event that the number of light emitting diodes is increased, it becomes difficult to design the backlight unit, in addition to the increase of production cost of the backlight unit.
Although nitride semiconductor-based light emitting diodes and InGaAlP-based light emitting diodes have been developed for the purpose of enhancing the light emitting efficiency, they tend to emit a light whose flux is lower than that of a cold cathode fluorescent lamp and therefore are not suitable for use in a backlight unit.